The present invention generally relates to convergence correction apparatuses, and more particularly to a convergence correction apparatus for correcting a misconvergence pattern generated in a self-convergence system color picture tube comprising in-line type electron guns.
Generally, in a color picture tube used in a color television receiver, electron beams emitted from three electron guns must not only be focused on a fluorescent screen, but the electron beams must also be converged in the same aperture of a shadow mask. For this reason, in the conventional color picture tube comprising the in-line type electron guns, a horizontal deflection magnetic field generated by a deflection yoke is of a strong pincushion type and a vertical deflection magnetic field generated by the deflection yoke is of a strong barrel type in order to obtain a satisfactory focusing of the three electron beams on the fluorescent screen and accordingly obtain a satisfactory convergence.
However, in a case where the deflection angle of the color picture tube is set to a large angle such as 90.degree., a pincushion distortion or a barrel distortion is generated in upper and lower rasters when the deflection magnetic fields are adjusted so as to obtain a satisfactory convergence. On the other hand, when the distortions in the upper and lower rasters are adjusted within a tolerable range, a positive cross misconvergence (hereinafter referred to as a positive trilemma misconvergence) or a negative cross misconvergence (hereinafter referred to as a negative trilemma misconvergence) is generated. Accordingly, it is virtually impossible or extremely difficult technically to simultaneously obtain a satisfactory convergence characteristic and tolerable distortions in the upper and lower rasters.
Accordingly, convergence correction apparatuses have been previously proposed in Japanese Laid-Open Patent Application Nos. 57-206184 and 58-14453 in which the applicant is the same as the assignee of the present application. The proposed convergence correction apparatuses comprise saddle-shaped horizontal deflecting coils and toroidal vertical deflecting coils which constitute the deflection yoke, and a reactor which changes its impedance with a vertical deflection period and is coupled to each of the horizontal deflecting coils. The circuit impedance of the horizontal deflecting coils is varied differentially by the reactors so as to correct the positive or negative trilemma misconvergence pattern by changing the horizontal deflection magnetic field distribution with time.
However, even in the proposed convergence correction apparatuses, there are problems in that the misconvergence pattern including the positive trilemma misconvergence pattern generated due to insufficient convergence correction at the central portion of the screen and the negative trilemma misconvergence pattern generated due to excess convergence correction at the peripheral portion of the screen is generated. It is extremely difficult to correct this kind of a misconvergence pattern by the conventional technology for the following reasons.
As a method of correcting the misconvergence pattern described above, there is a method disclosed in the Japanese Laid-Open Patent Application No. 57-206184 described before. According to this method, a D.C. magnetic bias quantity of permanent magnets is made small, and an operating range of control magnetic fields acting on the reactors is shifted accordingly. In order to obtain such an operating range, it is not only necessary to make the D.C. magnetic bias quantity small, but it is also necessary to make the diameters of drum-shaped cores which constitute the reactors considerably small. As a result, there are problems in that the yield of the drum-shaped cores becomes poor, the drum-shaped cores break easily when the coils are wound thereon, and it is difficult to mass produce the reactors.
Presently, it is impossible to completely eliminate the misconvergence described heretofore. For this reason, the color television receivers are produced by distributing the convergence error throughout the entire screen and performing adjustments so that the error appears on the average at each point of the screen. On the other hand, in a super or high fineness (resolution) picture tubes having a dot pitch of 0.21 to 0.31 mm on the screen, it is ideally desirable to make the convergence error to within 0.2 mm. However, it requires a highly skilled operator to adjust the convergence error to within such a small value, and it takes an extremely long time to make such an adjustment. Furthermore, it is extremely difficult to make such adjustments and constantly produce deflection yokes of satisfactory quality. In other words, it is extremely difficult to mass produce inexpensive deflection yokes which satisfy the demand for the small misconvergence.